The present invention relates to semiconductor device manufacturing, and more particularly to a method and apparatus for monitoring the process state of a semiconductor device fabrication process.
Within the semiconductor industry, an ever present need exists for improved process repeatability and control. For example, during the formation of a typical metal-layer-to-metal-layer interconnect a dielectric layer is deposited over a first metal layer, a via hole is etched in the dielectric layer to expose the first metal layer, the via hole is filled with a metal plug and a second metal layer is deposited over the metal plug (e.g., forming an interconnect between the first and the second metal layers). To ensure the interconnect has low contact resistance, all dielectric material within the via hole must be etched from the top surface of the first metal layer prior to formation of the metal plug thereon; otherwise, residual high-resistivity dielectric material within the via hole significantly degrades the contact resistance of the interconnect. Similar process control is required during the etching of metal layers (e.g., Al, Cu, Pt, etc.), polysilicon layers and the like.
Conventional monitoring techniques provide only a rough estimate of when a material layer has been completely etched (i.e., endpoint). Accordingly, to accommodate varying thicknesses of material layers (e.g., device variations) or varying etch rates of material layers (e.g., process/process chamber variations), an etch process may be continued for a time greater than a predicted time for etching the material layer (i.e., for an over-etch time). Etching for an over-etch time ensures that all material to be removed is removed despite device variations that increase the required etch time and despite process/process chamber variations which slow etch rate (and thus increase the required etch time).
While over-etch times ensure complete etching, over-etching increases the time required to process each semiconductor wafer and thus decreases wafer throughput. Further, the drive for higher performance integrated circuits requires each generation of semiconductor devices to have finer dimensional tolerances, making over-etching increasingly undesirable. A more attractive solution is a monitoring technique that identifies the causes of device variations and process/process chamber variations (e.g., chamber faults, improper reaction chemistries, improper etch rates, etc.) and that more accurately identifies processing events such as endpoint. However, no conventional monitoring technique provides sufficient information to serve both as a diagnostic tool that identifies deleterious process/process chamber variations and as a device processing control tool that tracks process progress accurately enough to reduce over-etch or other over-processing times required to compensate for both process/process chamber variations and device variations (e.g., material layer thickness variations, etch property variations, etc.).
Accordingly, a need exists for an improved method and apparatus for monitoring semiconductor device fabrication processes.
The present inventors have discovered that during a plasma process certain plasma xe2x80x9cattributesxe2x80x9d such as a plasma""s electromagnetic emissions or the RF power delivered to a wafer pedestal manifest low frequency fluctuations that contain significant information about the plasma process and the plasma chamber. For example, the intensity fluctuations of a plasma""s electromagnetic emissions (hereinafter xe2x80x9cplasma emission fluctuationsxe2x80x9d) have been found to contain information that falls within three broad categories:
(1) process state information such as plasma etch rate, RF power, wafer damage, wafer temperature, etch uniformity, plasma reaction chemistry, etc.;
(2) process event information such as when a particular material has been etched through or away (i.e., breakthrough), when a wafer is improperly held (i.e., improper xe2x80x9cchuckingxe2x80x9d), etc.; and
(3) process chamber information such as whether a chamber contains a fault, whether a chamber""s operation is similar to its previous operation or to another chamber""s operation (i.e., chamber matching), etc.
Similar information has been found within the fluctuations of the RF power delivered to a wafer pedestal during plasma processing.
To monitor plasma emission fluctuations, electromagnetic emissions generated by a plasma are collected, and a detection signal having at least one frequency component (having a magnitude associated therewith) is generated based on the intensity of the collected electromagnetic emissions. The magnitude of at least one frequency component of the detection signal then is monitored over time. Preferably frequency components having frequencies less than the RF frequency used to generate the plasma (e.g., 13.56 MHz), and most preferably less than about 50 kHz, are monitored over time. The preferred collected electromagnetic emissions comprise emissions having wavelengths within the range of about 200 to 1100 nanometers (i.e., broadband optical electromagnetic emissions), although other ranges may be employed. The electromagnetic emissions of particular chemical species associated with the plasma process (e.g., Al, AlCl, or BCl for an aluminum etch process) also may be monitored.
To monitor RF power fluctuations, the RF power (e.g., forward and/or reflected) delivered to a wafer pedestal during plasma processing is monitored and serves as the xe2x80x9cdetectionxe2x80x9d signal. The magnitude of at least one frequency component of the detection signal then is monitored over time. Preferably frequency components having frequencies less than the RF frequency used to generate the plasma (e.g., 13.56 MHz), and most preferably less than about 50 kHz, are monitored over time.
By monitoring the magnitude of at least one frequency component of a plasma emission fluctuation detection signal or of an RF power fluctuation detection signal over time, a characteristic fingerprint of a plasma process is obtained. The present inventors have discovered that features (e.g., the magnitude of frequency components and the position in time (xe2x80x9ctemporal positionxe2x80x9d) of frequency components) within a characteristic fingerprint provide process state information, process event information and process chamber information. These features may be monitored after a plasma process is performed or during the plasma process to allow for real-time process state control. In general, any chemical reaction having an attribute that varies with reaction rate may be similarly monitored (e.g., whether or not a plasma is employed and whether or not related to semiconductor device fabrication).